Acyl-CoA is a group of

coenzyme A cofactor is a non-protein chemical compound or metallic ion that is required for an enzyme's role as a catalyst (a catalyst is a substance that increases the rate of a chemical reaction). Cofactors can be considered "helper molecules" that ass ...

s that metabolize

fatty acid In chemistry, particularly in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, fr ...

s. Acyl-CoA's are susceptible to

beta oxidation In biochemistry and metabolism, beta-oxidation is the catabolic process by which fatty acid molecules are broken down in the cytosol in prokaryotes and in the mitochondria in eukaryotes to generate acetyl-CoA, which enters the citric acid cycl ...

, forming, ultimately,

acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for ...

. The acetyl-CoA enters the

citric acid cycle The citric acid cycle (CAC)—also known as the Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins ...

, eventually forming several equivalents of ATP. In this way, fats are converted to ATP, the universal biochemical energy carrier.

Functions

Fatty acid activation

Fats are broken down by conversion to acyl-CoA. This conversion is one response to high energy demands such as exercise. The oxidative degradation of fatty acids is a two-step process, catalyzed by acyl-CoA synthetase. Fatty acids are converted to their acyl phosphate, the precursor to acyl-CoA. The latter conversion is mediated by acyl-CoA synthase" :acyl-P + HS-CoA → acyl-S-CoA + P_i + H⁺ Three types of acyl-CoA synthases are employed, depending on the chain length of the fatty acid. For example, the substrates for medium chain acyl-CoA synthase are 4-11 carbon fatty acids. The enzyme acyl-CoA thioesterase takes of the acyl-CoA to form a free fatty acid and coenzyme A.

Beta Oxidation of Acyl-CoA

The second step of fatty acid degradation is beta oxidation. Beta oxidation occurs in mitochondria. After formation in the cytosol, acyl-CoA is transported into the mitochondria, the locus of beta oxidation. Transport of acyl-CoA into the mitochondria requires carnitine palmitoyltransferase 1 (CPT1), which converts acyl-CoA into acylcarnitine, which gets transported into the mitochondrial matrix. Once in the matrix, acylcarnitine is converted back to acyl-CoA by CPT2. Beta oxidation may begin now that Acyl-CoA is in the mitochondria. Beta oxidation of acyl-CoA occurs in four steps. 1.

Acyl-CoA dehydrogenase Acyl-CoA dehydrogenases (ACADs) are a class of enzymes that function to catalyze the initial step in each cycle of fatty acid β-oxidation in the mitochondria of cells. Their action results in the introduction of a trans double-bond between C2 ...

catalyzes dehydrogenation of the acyl-CoA, creating a double bond between the alpha and beta carbons.

FAD A fad or trend is any form of collective behavior that develops within a culture, a generation or social group in which a group of people enthusiastically follow an impulse for a short period. Fads are objects or behaviors that achieve short- ...

is the hydrogen acceptor, yielding FADH2. 2. Enoyl-CoA hydrase catalyzes the addition of water across the newly formed double bond to make an alcohol. 3. 3-hydroxyacyl-CoA dehydrogenase oxidizes the alcohol group to a ketone. NADH is produced from NAD+. 4.

Thiolase Thiolases, also known as acetyl-coenzyme A acetyltransferases (ACAT), are enzymes which convert two units of acetyl-CoA to acetoacetyl CoA in the mevalonate pathway. Thiolases are ubiquitous enzymes that have key roles in many vital biochemi ...

cleaves between the

alpha carbon In the nomenclature of organic chemistry, a locant is a term to indicate the position of a functional group or substituent within a molecule. Numeric locants The International Union of Pure and Applied Chemistry (IUPAC) recommends the us ...

and ketone to release one molecule of

Acetyl-CoA Acetyl-CoA (acetyl coenzyme A) is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle (Krebs cycle) to be oxidized for ...

and the Acyl-CoA which is now 2 carbons shorter. This four step process repeats until acyl-CoA has removed all carbons from the chain, leaving only Acetyl-CoA. During one cycle of beta oxidation, Acyl-CoA creates one molecule of Acetyl-CoA, FADH2, and NADH. Acetyl-CoA is then used in the

while FADH2 and NADH are sent to the

electron transport chain An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...

. These intermediates all end up providing energy for the body as they are ultimately converted to ATP. Beta Oxidation Process

Beta oxidation, as well as alpha-oxidation, also occurs in the

peroxisome A peroxisome () is a membrane-bound organelle, a type of microbody, found in the cytoplasm of virtually all eukaryotic cells. Peroxisomes are oxidative organelles. Frequently, molecular oxygen serves as a co-substrate, from which hydrogen pero ...

. The peroxisome handles beta oxidation of fatty acids that have more than 20 carbons in their chain because the peroxisome contains very-long-chain Acyl-CoA synthetases. These enzymes are better equipped to oxidize Acyl-CoA with long chains that the mitochondria cannot handle.

Example using Stearic Acid

Beta oxidation removes 2 carbons at a time, so in the oxidation of an 18 carbon fatty acid such as

Stearic Acid Stearic acid ( , ) is a saturated fatty acid with an 18-carbon chain. The IUPAC name is octadecanoic acid. It is a waxy solid and its chemical formula is C17H35CO2H. Its name comes from the Greek word στέαρ "''stéar''", which means tallow. ...

8 cycles will need to occur to completely break down Acyl-CoA. This will produce 9 Acetyl-CoA that have 2 carbons each, 8 FADH2, and 8 NADH.

Clinical Significance

Heart muscle primarily metabolizes fat for energy and Acyl-CoA metabolism has been identified as a critical molecule in early stage heart muscle pump failure. Cellular acyl-CoA content correlates with insulin resistance, suggesting that it can mediate lipotoxicity in non-adipose tissues. Acyl-CoA: diacylglycerol acyltransferase (DGAT) plays an important role in energy metabolism on account of key enzyme in triglyceride biosynthesis. The synthetic role of DGAT in adipose tissue such as the liver and the intestine, sites where endogenous levels of its activity and triglyceride synthesis are high and comparatively clear. Also, any changes in the activity levels might cause changes in systemic insulin sensitivity and energy homeostasis. A rare disease called multiple acyl-CoA dehydrogenase deficiency (MADD) is a fatty acid metabolism disorder. Acyl-CoA is important because this enzyme helps make Acyl-CoA from free fatty acids, and this activates the fatty acid to be metabolized. This compromised fatty acid oxidation leads to many different symptoms, including severe symptoms such as cardiomyopathy and liver disease and mild symptoms such as episodic metabolic decomposition, muscle weakness and respiratory failure. MADD is a genetic disorder, caused by a mutation in the ETFA, ETFB, and ETFDH genes. MADD is known as an "autosomal recessive disorder" because for one to get this disorder, one must receive this recessive gene from both parents.

References

External links

* {{Fatty-acid metabolism intermediates Metabolism Thioesters of coenzyme A